The present invention relates to a semiconductor device of a multilayer wiring structure and, more particularly, to an improvement in a multilayer wiring structure.
A semiconductor device of a multilayer wiring structure in which a multilayer comprising two insulation films is used as an insulating interlayer and a polyimide-based resin film is used as a final passivation film is conventionally known.
An example of a semiconductor device having such a two-layer wiring structure as a multilayer wiring structure (i.e., having two metal wiring layers) and a manufacturing process thereof will be described with reference to FIGS. 1 to 3.
Referring to FIG. 1, a first metal wiring layer 13 formed of an aluminum-silicon alloy has a predetermined pattern and is formed on a semiconductor substrate (e.g., silicon substrate) 11 through a first insulation film 12, such as an SiO.sub.2 film. Then, a silicon nitride film 14, to be a main insulating interlayer of a multilayer insulation film, is formed to have a thickness of about 0.8 .mu.m on the resultant substrate by a plasma chemical vapour deposition method within a temperature range that the first metal wiring layer 13 can withstand. In this case, a portion of the silicon nitride film 14 formed on the edge of the first metal wiring layer 13 has a recessed portion having a steep step A. An SiO.sub.2 -based resin film consisting of, e.g., "OCD .RTM." (where .RTM. means a trade name) is deposited on the silicon nitride film 14, and thereafter a silica film 15 of a thickness of about 0.6 .mu.m, to be a subinsulating interlayer of the multilayer insulation film, is formed. Then, the silica film 15 is heat-treated to be hardened. This silica film (insulation film) 15 is formed to moderate the steep step A of the silicon nitride film 14 serving as the main insulating interlayer. Therefore, when a liquid material is deposited on the silicon nitride film 14, it fills a portion having the steep step A of the film 14 and so the step is moderated.
Referring to FIG. 2, the multilayer insulation film consisting of the silicon nitride film 14 and the silica film 15 is patterned by a conventional photolithography method (e.g., reactive ion etching (RIE)), and a contact hole 101 or the like is formed. A photoresist film (not shown) for patterning is removed by plasma etching using O.sub.2 gas.
Referring to FIG. 3, a second metal wiring layer 16 consisting of pure aluminum is formed on the resultant substrate. Thereafter, a polyimide-based resin solution is spin-coated and is then heat-treated to be hardened. As a result, a polyimide-based resin film 17 as a final passivation film is formed to a thickness of about 2.0 .mu.m. Then, the polyimide-based resin film 17 is patterned by a conventional photolithography method using a hydrazine hydrate-based etching solution such that an opening (not shown) for forming a bonding pad is formed. A photoresist film (not shown) for patterning is removed by, for example, an organic photoresist solution and then the final heat-treatment is performed.
In this manner, a conventional semiconductor device of a two-layer wiring structure is formed.
However, the conventional semiconductor device described above has the following problems.
The silicon nitride film as the main insulating interlayer of a multilayer insulation film is formed by a plasma CVD method at a relatively low temperature, such that a first metal wiring layer mainly consisting of aluminum and underlying the multilayer insulation film is not damaged. In this case, a portion of the silicon nitride film 14, which is formed on the edge of the underlying metal wiring layer 13, has a steep step indicated by the reference symbol "A" in FIG. 1. If the second metal wiring layer 16 is directly formed on this silicon nitride film 14 having the steep step A, there is a possibility that step coverage will be poor in a portion of the second metal wiring layer 16 formed on the step A. Therefore, the silica solution is deposited to moderate the slope of the step A as described above. In other words, the silica solution fills a portion having the step and so the step is moderated. The silica solution forms the silica film 15 as the subinsulating interlayer. Then, the second metal wiring layer 16 is formed on this subinsulating interlayer, thereby preventing poor step coverage thereof.
In a multilayer wiring structure having such a subinsulating interlayer, a polyimide-based resin film is formed as the final passivation film 17 as described above. However, it is preferable that an inorganic hard insulating substance such as Si.sub.3 N.sub.4, which has good humidity resistance, impurity blocking properties and resistance to damage, is used as a final passivation film. However, such an insulating substance cannot be used in the conventional device described above. The silica film 15 formed as the subinsulating interlayer is soft in comparison with an inorganic insulation film, such as an Si.sub.3 N.sub.4 film, and has poor adhesion with respect to the inorganic insulation film, such as the Si.sub.3 N.sub.4 film. For this reason, if the hard inorganic insulation film, such as the Si.sub.3 N.sub.4 film, is formed on the soft silica film 15, the inorganic insulation film tends to be shifted slightly. As a result, cracks are formed in the inorganic insulation film by a stress acting thereon. In particular, when a semiconductor chip having such a configuration is encapusulated by resin molding, high stress, caused by a variation in temperature in a molding resin, acts on a passivation film and cracks are formed.
For the same reason, when such a conventional semiconductor device has three or more metal wiring layers, an inorganic hard insulation film cannot be used as an insulating interlayer except as a first insulating interlayer. If the hard inorganic insulation film is used in addition to the first insulating interlayer, the hard inorganic insulation film is shifted for the same reason described above, and cracks are formed therein. Furthermore, when the conventional semiconductor device described above undergoes cracking in the inorganic insulation film, satisfactory insulation for the metal wiring layer formed thereon cannot be provided.